This invention relates to an improved process for the conversion of aromatic hydrocarbons, such as conversion of toluene into paraxylene. More specifically, the present invention concerns operating a disproportionation process at very low hydrogen to hydrocarbon level to promote favorable selectivity.
The xylene isomers are produced in large volumes from petroleum as feedstocks for a variety of important industrial chemicals. The most important of the xylene isomers is para-xylene, the principal feedstock for polyester which continues to enjoy a high growth rate from large base demand. Orthoxylene is used to produce phthalic anhydride, which has high-volume but mature markets. Meta-xylene is used in lesser but growing volumes for such products as plasticizers, azo dyes and wood preservers. Ethylbenzene generally is present in xylene mixtures and is occasionally recovered for styrene production, but usually is considered a less-desirable component of C8 aromatics.
Among the aromatic hydrocarbons, the overall importance of the xylenes rivals that of benzene as a feedstock for industrial chemicals. Neither the xylenes nor benzene are produced from petroleum by the reforming of naphtha in sufficient volume to meet demand, and conversion of other hydrocarbons is necessary to increase the yield of xylenes and benzene. Often toluene is selectively disproportionated to yield benzene and C8 aromatics from which the individual xylene isomers are recovered.
A current objective of many petrochemical and aromatics complexes is to increase the yield of xylenes and to de-emphasize benzene production Demand is growing faster for xylene derivatives than for benzene derivatives. Refinery modifications are being effected to reduce the benzene content of gasoline in industrialized countries, which will increase the supply of benzene available to meet demand. A higher yield of xylenes at the expense of benzene thus is a favorable objective, and processes to transalkylate C9 aromatics along with toluene have been commercialized to obtain high xylene yields.
U.S. Pat. No. 4,016,219 B1 (Kaeding) discloses a process for toluene disproportionation using a catalyst comprising a zeolite which has been modified by the addition of phosphorus in an amount of at least 0.5 mass-%. The crystals of the zeolite are contacted with a phosphorus compound to effect reaction of the zeolite and phosphorus compound. The modified zeolite then may be incorporated into indicated matrix materials.
U.S. Pat. No. 4,097,543 B1 (Haag et al.) teaches toluene disproportionation for the selective production of para-xylene using a zeolite which has undergone controlled precoking. The zeolite may be ion-exchanged with a variety of elements from Group IB to VIII, and composited with a variety of clays and other porous matrix materials.
U.S. Pat. No. 4,182,923 B1 (Chu) describes a process for toluene disproportionation with a high conversion of the toluene to benzene and paraxylene by use of an aluminosilicate zeolite of silica to alumina ratio above 12 and which has been modified by treatment with ammonium hydrogen phosphate to deposit phosphorus.
U.S. Pat. No. 4,629,717 B1 (Chao) discloses a phosphorus-modified alumina hydrogel formed by gelation of a homogeneous hydrosol. The composite has a high surface area of 140-450 m2/g and high activity and selectivity in 1-heptene conversion tests.
U.S. Pat. No. 6,114,592 B1 (Gajda et al.), which is incorporated herein by reference, teaches an improved process combination for the selective disproportionation of toluene. The combination comprises selective hydrogenation of a toluene feedstock followed by a zeolitic catalyst.
Workers in the field of aromatics disproportionation continue to seek processes and catalysts having exceptionally high selectivity for paraxylene from toluene combined with favorable activity and stability.
It is an object of the present invention to provide an improved process for the disproportionation of aromatic hydrocarbons. A specific objective is to obtain a high yield of xylenes by selective toluene disproportionation.
This invention is based on the unexpected finding that operation at very low levels of hydrogen to hydrocarbon promotes favorable selectivity to paraxylene. Soft coke that is deposited upon a catalyst at such low ratio processing can also be periodically removed by conducting periodic rejuvenation at higher molar ratios of hydrogen to hydrocarbon. Further, low ratio processing also improves the selective precoking and conditioning of zeolitic catalysts.
The present invention is directed to a process for the production of xylene comprising a selective disproportionation zone at conditions comprising a hydrogen to hydrocarbon ratio of less than 0.5. In the disproportionation zone the stream is contacted with a disproportionation catalyst at disproportionation conditions. The disproportionation catalyst preferably comprises a pentasil zeolitic aluminosilicate, most preferably MFI. This catalyst is subjected to a precoking step prior to its use in the disproportionation zone in order to increase its selectivity to paraxylene in the product beyond its equilibrium concentration.
Mixed C8 aromatics recovered from the disproportionation effluent are sent to a xylene-separation zone; preferably, para-xylene is recovered by adsorption and/or crystallization. The xylene-separation zone may also comprise a catalytic alkyl-aromatic zone for ethylbenzene conversion and dealkylation.
Periodic disproportionation catalyst rejuvenation can also be conducted by increasing the ratio of hydrogen to hydrocarbon above 0.5, preferably to somewhere in the range of about 1 to about 5.
These as well as other objects and embodiments will become apparent from the detailed description of the invention.